Bone cement plugs are well known in the art. Such devices are generally used in conjunction with bone cement dispensers to compact bone cement into a bone canal before fixing a prosthetic device in that bone canal. By way of example, bone cement plugs are commonly used in conjunction with bone cement dispensers to compact bone cement into the intramedullary canal of the femur before fixing the femoral stem of an artificial hip in that canal.
More particularly, in total joint replacement surgeries such as hip and shoulder replacements, bone cement is commonly used to fix the stems of the prosthetic devices into the intramedullary canals of the joint's bones. In this respect it has generally been found that a prosthetic device will be more securely fixed in a bone canal if the bone cement is well packed into the bone canal before the distal end of the prosthetic device is positioned in the bone canal.
To this end, after initial preparation and cleaning of the bone canal, the distal end of the canal is occluded with a plug. The bone cement plug serves to limit uncontrolled flow of bone cement into the distal portion of the bone canal. Ideally, the bone cement plug limits the column of bone cement to about 1 to 2 cm beyond the distal tip of the stem of the prosthesis. After the plug has been set at the distal end of the bone canal, the bone cement is then injected into the distal-most part of the bone canal, adjacent to the plug, using a bone cement dispenser having a long nozzle. The bone canal is then filled with bone cement in a retrograde fashion, by withdrawing the nozzle of the bone cement dispenser from the distal end of the bone canal to the proximal end of the bone canal as the cement issues from the nozzle. Such retrograde filling helps avoid trapping air in the distal-most part of the bone canal.
After the bone canal has been filled with bone cement, a bone canal pressurizer is then connected to the bone cement dispenser. The pressurizer is pressed against the open end of the bone so as to occlude the bone canal. More cement is then injected into the bone canal through the pressurizer and under pressure. Under such pressurization, the cement in the bone canal intrudes into the interstices of the inner surface of the bone canal. When the bone cement thereafter sets, a micro-interlock is established between the cement and the irregularities of the interior surface of the bone canal. This significantly enhances fixation of the prosthetic device in the bone canal.
Ideally, a bone cement plug should be easy to deploy at the distal end of the bone canal, effective in closing off that bone canal and, in the event that the bone cement plug subsequently needs to be removed, easy to retrieve from the distal end of the bone canal. The bone cement plug must also be bio-compatible with the patient. Furthermore, the bone cement plug should be inexpensive to produce.
A variety of bone cement plugs are known in the art. See, for example, the bone cement plugs described and illustrated in U.S. Pat. Nos. 4,245,359; 4,276,659; 4,293,962; 4,302,855; 4,344,190; 4,447,915; 4,627,434; 4,686,973; 4,697,584; 4,745,914; 4,936,859; 4,950,295; 4,994,085; 5,061,287; 5,078,746; 5,092,891; 5,376,120; and 5,383,932. See also, for example, the bone cement plug described and illustrated in British Patent Document No. 2,253,564A. See also, for example, the publication entitled "Polyethylene medullary plug according to Stuhmer/Weber" distributed by ALLO PRO AG of Switzerland. See also related apparatus described and illustrated in U.S. Pat. Nos. 4,011,602; 4,523,587; and 4,904,267. See also related apparatus described and illustrated in European Patent Document No. 0,006,408 B1; and PCT Patent Document No. WO 94/15544.
Unfortunately, however, all of the bone cement plugs developed to date tend to suffer from one or more significant disadvantages.
More particularly, in general, the fixation of the bone cement plug depends on the friction between the inner wall of the bone canal and the bone cement plug. Currently, the most common surgical technique is to first measure the size of the prepared bone canal. This involves sequentially inserting a number of "sizers" into the bone canal so as to determine the gross cross-sectional diameter of the canal at the desired depth. Having thus determined the size of the canal, an over-sized plug is inserted into the canal so as to occlude the distal portion of the bone canal. If the bone cement plug is not sufficiently over-sized, or if the design of the plug is overly-deformable, the bone cement plug's engagement with the inner wall of the bone canal will be less than optimal. During the pressurization phase or during insertion of the prosthesis, the increased pressure of the bone cement can then cause the insufficiently-anchored bone cement plug to migrate distally. However, if the bone cement plug is greatly over-sized, and/or if the design of the bone cement plug is overly rigid, the bone cement plug cannot be inserted into the bone canal to the desired depth. Excessively forceful insertion of the bone cement plug can cause the wall of the bone canal to fracture.
Many of the current bone cement plugs cannot adequately anchor against the wall of the bone canal located beyond the isthmus, i.e., against that portion of the bone canal located beyond the narrowest part of the canal. This is because the largest size of the bone cement plug is limited by the need for the bone cement plug to pass through the narrowest part of the canal. In other words, a smaller than desired plug size is chosen.
Various expandable plugs are designed to addressed the above issues. However, such known bone cement plugs suffer from a number of drawbacks such as difficulty in manufacturing, inadequate fixation, the complexity of the insertion tools, etc.